Single-electrode capacitive sensors for sensing the proximate presence of an object are commonly used as non-contact electric switches (e.g., elevator call buttons), as part of a water valve controller for operating a wash basin valve in a public restroom, for monitoring the flow of objects on an assembly line and the like. Sensors of this sort are commonly configured to provide a binary output and operate by measuring a value of electrical capacitance to an electric ground. If the sensor is configured as a "proximity sensor" it provides an output determinative of proximate presence when the value of the measured capacitance exceeds a predetermined threshold valve. If the sensor is configured as a "motion sensor" it provides the determinative output when the rate of change of capacitance exceeds a predetermined threshold value.
Multi-electrode capacitive sensors having a plurality of electrodes disposed about a predetermined sensing area can determine the position of an object adjacent the area and, by making multiple measurements over a period of time, can determine the direction and speed of motion of the object. Although there are many possible applications for multi-electrode capacitive sensors (e.g., for controlling the intensity of a light or sound source responsive to an operator moving his or her hand across the sensing area), the application that appears to be of greatest interest is that of providing an input to a computer that uses the input to determine the position of a cursor on an associated display.
Over the past decade, pointing devices have become mandatory instruments for dealing with the increasingly graphical nature of computer user interfaces. At the same time, small, portable, battery-powered `laptop` or `notebook` computers have become increasingly popular. This popularity of laptop computers has produced a need for new pointing devices of increasingly small size to cope with the dramatic reduction in available keyboard and surrounding workspace.
Popular laptop pointing devices include inter alia (a) small trackballs on or adjacent the keyboard, b) tiny joysticks interposed among the keyboard keys, (c) capacitive touch pads, and (d) accessory mice to be used near the laptop, but not built into it. The joystick has been found wanting by many, as it is a limited movement `velocity` device. That is, small amounts of force applied laterally to the stick cause the pointer to accelerate to a velocity proportional to the force and to move in the direction of the applied force. This less than convenient method requires considerable accommodation on the part of the user. Capacitive finger touch pads, which may share the same housing as the keyboard but which do not share an active typing area of the keyboard, offer a significant improvement by presenting a more substantial surface area to the user, and allowing movements that are generally more intuitive than those of a joystick. Neither method has the large usable area of a conventional mouse placed on an adjacent desk surface. The touch pad device also negatively affects the available keyboard layout, as it requires a large area in front of the keys. Users also report that their thumbs can accidentally strike the pad while typing, thus dislodging the pointer from its prior position, causing frustration.
Ng et al., in U.S. Pat. No. 4,476,463, disclose a pointing device responsive to the position of a user's finger adjacent a computer display overlaid with a resistive sheet having three or more electrodes spaced thereabout. In one embodiment, the position of the user's finger is determined from a sequence of capacitive measurements, each of which involves charging the sheet and then transferring charge from the sheet into an A to D converter. In a burst mode of operation this charge/discharge/measure sequence may be repeated a predetermined number of times to obtain an average value from which a distance (relative to the one electrode at which the measurement is taken) is determined. After all the electrodes have been interrogated, the position is determined (or over-determined) as the point satisfying all the distance criteria. Ng et al.'s teaching embraces an arrangement for resetting reference levels in the capacitive measurement to adapt to environmental changes.
Yoshikawa et al., in U.S. Pat. No. 4,680,430, teach a pointing device responsive to the position of a user's finger (which is capacitively coupled to ground) above a uniformly resistive layer of material. Yoshikawa et al. teach the use of their device as a coarse-resolution touch-screen input device, but do not disclose integrating their pointing device with a keyboard or keypad.
Matzke et al., in U.S. Pat. No. 4,736,191, teach a multi-electrode capacitive pointing device comprising an array of electrodes symmetrically disposed about a central point. Their array, which is operated by sequentially charging and discharging the electrodes to capacitively determine the position of a user's finger adjacent the device, is disposed near the keyboard of a computer.
Boie et al, in U.S. Pat. No. 5,463,388, teach a computer mouse or keyboard input device utilizing capacitive sensors. Their input device employs an array of metallic electrodes and detects the position of a user's finger from a centroid of capacitance values. The position information is used to either move the cursor (if the apparatus is being used as a pointing device) or to select a character to be input (if the apparatus is being used as a keyboard). Boie et al. do not disclose integrating their pointing device with a keyboard or a keypad.
Brandenburg et al., in U.S. Pat. No. 5,499,041, teach a keyswitch-integrated pointing device responsive to changes in the force and in the balance of forces applied to a resistor array made from a sheet of a resistive polymeric material. Their resistor array is associated with a single key of a plurality of keys in a keyboard, each of the keys having a respective one of an array of parallel plungers associated therewith. Their device is usable for pointing purposes only after their modified key has been pushed down to its lowermost position.
Selker et al., in U.S. Pat. No. 5,521,596, teach the use of a joystick disposed in the active typing area of a computer keyboard as a pointing device.
Gerpheide et al., in U.S. Pat. No. 5,565,658, teach a touch-pad capacitance-based computer pointing device using two orthogonal arrays of metallic stripes separated by an insulator. They employ an arrangement in which the capacitance measurement is made synchronously with a reference frequency selected to not be coherent with the interfering signal.
The inventor, in his U.S. patent, U.S. Pat. No. 5,730,165, the disclosure of which is herein incorporated by refrence, teaches a capacitive field sensor employing a single coupling plate to detect change in capacitance to ground. This apparatus comprises both a circuit for charging a sensing electrode as well as a switching element to remove charge from the sensing electrode and to transfer it to a charge detection circuit.
Baxter, in a book entitled , "Capacitive Sensors, Design and Applications" (IEEE Press 1997 ISBN 0-7803-1130-2.), describes a capacitive position measuring device having shaped electrodes and using a "slider" pickoff electrode. Baxter also describes the use of a resistive sheet to generate a electric field in a ratiometric position sensor employing a slider pick-off.